Satellite radiotelephone communications systems and methods are widely used for radiotelephone communications. Satellite radiotelephone communications systems and methods generally employ at least one space-based component, such as one or more satellites, that is/are configured to wirelessly communicate with a plurality of satellite radiotelephones.
A satellite radiotelephone communications system or method may utilize a single satellite antenna pattern (beam or cell) covering an entire service region served by the system. Alternatively, or in combination with the above, in cellular satellite radiotelephone communications systems and methods, multiple satellite antenna patterns (beams or cells) are provided, each of which can serve a substantially distinct service region in an overall service region, to collectively provide service to the overall service region. Thus, a cellular architecture that is similar to that used in conventional terrestrial cellular radiotelephone systems and methods can be implemented in cellular satellite-based systems and methods. The satellite typically communicates with radiotelephones over a bidirectional communications pathway, with radiotelephone communications signals being communicated from the satellite to the radiotelephone over a downlink or forward link (also referred to as forward service link), and from the radiotelephone to the satellite over an uplink or return link (also referred to as return service link). In some cases, such as, for example, in broadcasting, the satellite may communicate information to one or more radioterminals unidirectionally.
The overall design and operation of cellular satellite radiotelephone systems and methods are well known to those having skill in the art, and need not be described further herein. Moreover, as used herein, the term “radiotelephone” includes cellular and/or satellite radiotelephones with or without a multi-line display; Personal Communications System (PCS) terminals that may combine a radiotelephone with data processing, facsimile and/or data communications capabilities; Personal Digital Assistants (PDA) that can include a radio frequency transceiver and/or a pager, Internet/Intranet access, Web browser, organizer, calendar and/or a global positioning system (GPS) receiver; and/or conventional laptop and/or palmtop computers or other appliances, which include a radio frequency transceiver. A radiotelephone also may be referred to herein as a “radioterminal,” a “mobile terminal,” a “user device,” or simply as a “terminal”. As used herein, the term(s) “radioterminal,” “radiotelephone,” mobile terminal,” “user device” and/or “terminal” also include(s) any other radiating user device, equipment and/or source that may have time-varying or fixed geographic coordinates and/or may be portable, transportable, installed in a vehicle (aeronautical, maritime, or land-based) and/or situated and/or configured to operate locally and/or in a distributed fashion over one or more terrestrial and/or extra-terrestrial location(s). Furthermore, as used herein, the term “space-based component” or “space-based system” includes one or more satellites at any orbit (geostationary, substantially geostationary, medium earth orbit, low earth orbit, etc.) and/or one or more other objects and/or platforms (e.g., airplanes, balloons, unmanned vehicles, space crafts, missiles, etc.) that has/have a trajectory above the earth at any altitude.
Terrestrial networks can enhance cellular satellite radiotelephone system availability, efficiency and/or economic viability by terrestrially using/reusing at least some of the frequencies that are allocated to cellular satellite radiotelephone systems. In particular, it is known that it may be difficult for cellular satellite radiotelephone systems to reliably serve densely populated areas, because satellite signals may be blocked by high-rise structures and/or may not penetrate into buildings. As a result, satellite spectrum may be underutilized or unutilized in such areas. The terrestrial use/reuse of at least some of the satellite system frequencies can reduce or eliminate this potential problem.
Moreover, the capacity of an overall hybrid system, including space-based (i.e., satellite) and terrestrial communications capability, may be increased by the introduction of terrestrial frequency use/reuse of frequencies authorized for use by the space-based component, since terrestrial frequency use/reuse may be much denser than that of a satellite-only system. In fact, capacity may be enhanced where it may be mostly needed, i.e., in densely populated urban/industrial/commercial areas. As a result, the overall system may become more economically viable, as it may be able to serve more effectively and reliably a larger subscriber base.
One example of terrestrial reuse of satellite frequencies is described in U.S. Pat. No. 5,937,332 to inventor Karabinis entitled Satellite Telecommunications Repeaters and Retransmission Methods, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein. As described therein, satellite telecommunications repeaters are provided which receive, amplify, and locally retransmit the downlink/uplink signal received from a satellite/radioterminal thereby increasing an effective downlink/uplink margin in the vicinity of the satellite telecommunications repeater and allowing an increase in the penetration of uplink and downlink signals into buildings, foliage, transportation vehicles, and other objects which can reduce link margin. Both portable and non-portable repeaters are provided. See the abstract of U.S. Pat. No. 5,937,332. Satellite radiotelephones for a satellite radiotelephone system or method having a terrestrial communications capability by terrestrially using/reusing at least some frequencies of a satellite frequency band and using substantially the same air interface for both terrestrial and satellite communications may be more cost effective and/or aesthetically appealing compared to other alternatives. Conventional dual band/dual mode radiotelephone alternatives, such as the well known Thuraya, Iridium and/or Globalstar dual mode satellite/terrestrial radiotelephones, duplicate some components (as a result of the different frequency bands and/or air interface protocols between satellite and terrestrial communications), which leads to increased cost, size and/or weight of the radiotelephone. See U.S. Pat. No. 6,052,560 to inventor Karabinis, entitled Satellite System Utilizing a Plurality of Air Interface Standards and Method Employing Same.
Satellite radioterminal communications systems and methods that may employ terrestrial use and/or reuse of satellite frequencies by an Ancillary Terrestrial Network (ATN) including at least one Ancillary Terrestrial Component (ATC) are also described in U.S. Pat. No. 6,684,057 to Karabinis, entitled Systems and Methods for Terrestrial Reuse of Cellular Satellite Frequency Spectrum; U.S. Pat. No. 6,785,543 to Karabinis, entitled Filters for Combined Radiotelephone/GPS Terminals; U.S. Pat. No. 6,856,787 to Karabinis, entitled Wireless Communications Systems and Methods Using Satellite-Linked Remote Terminal Interface Subsystems; U.S. Pat. No. 6,859,652 to Karabinis et al., entitled Integrated or Autonomous System and Method of Satellite-Terrestrial Frequency Reuse Using Signal Attenuation and/or Blockage, Dynamic Assignment of Frequencies and/or Hysteresis; and U.S. Pat. No. 6,879,829 to Dutta et al., entitled Systems and Methods for Handover Between Space Based and Terrestrial Radioterminal Communications, and For Monitoring Terrestrially Reused Satellite Frequencies At a Radioterminal to Reduce Potential Interference, and in U.S. Pat. Nos. 6,892,068, 6,937,857, 6,999,720 and 7,006,789; and Published U.S. Patent Application Nos. US 2003/0054761 to Karabinis, entitled Spatial Guardbands for Terrestrial Reuse of Satellite Frequencies; US 2003/0054814 to Karabinis et al., entitled Systems and Methods for Monitoring Terrestrially Reused Satellite Frequencies to Reduce Potential Interference; US 2003/0073436 to Karabinis et al., entitled Additional Systems and Methods for Monitoring Terrestrially Reused Satellite Frequencies to Reduce Potential Interference; US 2003/0054762 to Karabinis, entitled Multi-Band/Multi-Mode Satellite Radiotelephone Communications Systems and Methods; US 2002/0041575 to Karabinis et al., entitled Coordinated Satellite-Terrestrial Frequency Reuse; US 2003/0068978 to Karabinis et al., entitled Space-Based Network Architectures for Satellite Radiotelephone Systems; US 2003/0153308 to Karabinis, entitled Staggered Sectorization for Terrestrial Reuse of Satellite Frequencies; and US 2003/0054815 to Karabinis, entitled Methods and Systems for Modifying Satellite Antenna Cell Patterns In Response to Terrestrial Reuse of Satellite Frequencies, and in Published U.S. Patent Application Nos. 2004/0121727, 2004/0142660, 2004/0192395, 2004/0192200, 2004/0192293, 2004/0203742, 2004/0240525, 2005/0026606, 2005/0037749, 2005/0041619, 2005/0064813, 2005/0079816, 2005/0090256, 2005/0118948, 2005/0136836, 2005/0164700, 2005/0164701, 2005/0170834, 2005/0181786, 2005/0201449, 2005/0208890, 2005/0221757, 2005/0227618, 2005/0239457, 2005/0239403, 2005/0239404, 2005/0239399, 2005/0245192, 2005/0260984, 2005/0260947, 2005/0265273, 2005/00272369, 2005/0282542, 2005/0288011, 2006/0040613, 2006/040657 and 2006/0040659; all of which are assigned to the assignee of the present invention, the disclosures of all of which are hereby incorporated herein by reference in their entirety as if set forth fully herein.
Owing to the greater capacity typical of terrestrial wireless networks, the potential exists for the number of terminals registered in a hybrid network including a satellite subnetwork component and an ancillary terrestrial component (ATC) subnetwork component, to far exceed the capacity of the satellite subnetwork. It is also known that the coverage of terrestrial networks has “coverage holes”, i.e. pockets of areas surrounded by covered regions, where there is insufficient signal strength from any base station to execute either an incoming or outgoing call, or both. An exemplary context of this invention is a hybrid wireless network, including a satellite subnetwork and a terrestrial subnetwork with roaming and handover allowed between the two subnetworks and where the coverage of the satellite subnetwork blankets that of the terrestrial subnetwork. In such a hybrid network, unless mitigating measures are taken, a significant number of mobile terminals passing through the coverage hole in the idle mode (when a call is not in progress) will roam to the satellite subnetwork on entering the coverage hole and roam back to the terrestrial network on leaving the coverage hole, assuming that terrestrial access is preferred to satellite access for quality-of-service and cost reasons.
If the two subnetworks are considered distinct Location Areas (LA's), this may involve an LA update. An LA update typically involves a registration in which the mobile terminal communicates with the subnetwork. This communication with the satellite subnetwork, from a large number of mobile terminals passing through a terrestrial coverage hole during a busy hour, may cause a substantial load to the satellite subnetwork, which is typically dimensioned to handing much less traffic, especially from a single spotbeam. This overload may occur even when most of the mobile terminals passing through the coverage hole may not actually engage in any traffic communication with the satellite subnetwork. On leaving the coverage hole, when the mobile terminal senses that terrestrial coverage is again available, it is generally desirable for the mobile terminal to deregister from the satellite subnetwork, which may involve further communication with the satellite subnetwork, and thereby add further load to the latter. For example, in the GSM protocol and its derivatives, such deregistration may include an IMSI Detach procedure. United States Patent Publication No. 2005-0090256, published Apr. 28, 2005, entitled “SYSTEMS AND METHODS FOR MOBILITY MANAGEMENT IN OVERLAID MOBILE COMMUNICATIONS SYSTEMS, and incorporated by reference herein, describes implicit and explicit registration techniques that can reduce such load on the satellite subnetwork.
Traditionally, handovers between Public Land Mobile Networks (PLMNs) have not been performed by the commercial cellular communications industry. Inter-PLMN idle mode roaming is common in conventional systems, but such roaming typically involves a time consuming authentication process designed to ensure that the visiting mobile terminal has the right credentials to receive service from the visited PLMN. Therefore, it may be undesirable to make the full idle mode roaming procedure a component of a handover procedure in a hybrid satellite/terrestrial system.
In GSM, call handover between one base transceiver station (BTS) to another BTS (regardless of whether they belong to the same LA or MSC), say from BTS-A to BTS-B, involves two levels of synchronization. At a first level, the mobile terminal is synchronized in frequency and phase to the forward control channel of BTS-B. This may be achieved by “sniffing” the forward control channels of adjacent cells periodically during idle periods in the TDMA frame. This may be done before the mobile terminal has made the transition to BTS-B and is referred to as pre-synchronization. At a second level, after the handover, the mobile terminal may re-adjust its TDMA frame timing advance to a new value that matches the propagation delay to BTS-B, which, in general, will be different from the propagation delay to BTS-A. This is referred to as post-handover synchronization, or simply post-synchronization, and may be performed synchronously or asynchronously.
In synchronous post-handover synchronization, the new timing advance is known to the mobile terminal a priori. A variety of techniques are used/allowed in legacy GSM systems with respect to acquiring this a priori information, as described, for example, in Michel Mouly and Marie-Bernadette Pautet, The GSM System for Mobile Communications, Cell & Sys, 1992, ISBN 2-9507190-0-7, pp. 347-349. The handshaking process in synchronous post-handover synchronization involves the MT sending a small number of access probes (referred to in GSM as RIL3-RR Handover Access messages) to the new BTS (BTS-B), which then activates the new channel, with the new timing advance, in both directions.
In asynchronous post-handover synchronization, no a priori information about the correct timing advance for communication between the MT and BTS-B is used. The correct timing advance is assessed by access probes sent by the MT to BTS-B, and the MT conventionally is forbidden from transmitting on the new channel until the new timing advance is unequivocally established, although reception may be allowed. Handshaking involved in such a process is discussed in Asha Mehrotra, GSM System Engineering, Artech House, 1997, ISBN 0-89006-860-7, pp. 147-148.